Trusted computing involves incorporating security measures into the core of a computing platform. Although administrators may achieve limited security using software-based security, administrators can only trust such software-based security to the extent they trust the hardware or software running the software-based security application. For example, a virus in the Master Boot Record (MBR) could load before the operating system and cause damage that remains undetected by any software-based security applications run by the operating system. To achieve trusted computing, an administrator needs confidence that a wrongdoer has not maliciously modified the software, firmware, and hardware since the last system boot. In other words, an administrator should feel confident that the combination of hardware and software that comprise the platform is operating properly and according to some specification. In order to compare the state of the platform to a specification, an administrator needs the ability to reliably measure and report information about the platform. This measured information is then validated to declare that the platform is exactly what it says it is and can be trusted for a given purpose. The goal of trusted computing is to provide confidence in the state of the data processing system from power-up to shutdown.
One method to provide administrators with such confidence is through the use of a separate, secure piece of hardware known as a Trusted Platform Module (TPM). A TPM could be made according to specifications recognized by the Trusted Computing Group (TCG). The TCG is an industry-standards organization that promotes security across computing platforms in personal computers (PCs), personal digital assistants (PDAs), and mobile phones, for example. TCG provides specifications for TPMs to allow computer administrators to assert ownership of a platform while allowing users the ability to perform job functions. The TPM specification defines a Core Root of Trust for Measurement (CRTM) to ensure compatibility and security among computing platforms. The TPM is also known as the “Hardware Root of Trust.”
A root of trust is a set of unconditionally trusted functions that serve as the foundation on which all other trust is built. Upon booting up a computer, the execution of the platform begins with execution of the CRTM. For a PC, the CRTM may either be the entire BIOS boot block or a portion of the BIOS boot block. The integrity of the CRTM is the foundation for the trust in the rest of the system after boot-up. Therefore, both the CRTM and TPM should be protected against hardware and software attacks.
The CRTM measures the integrity of software during boot-up and runtime. For a PC, the CRTM measures the code in the BIOS, MBR, and possibly other firmware. The CRTM can measure the code by performing a hash function on the code. These measurements reflect the state of the software at the time of the hash. The CRTM then reports the measurements to the TPM. Upon receiving the measurements, the TPM may compare the measurements to known values. If the measurements do not match the known values, the system may stop booting or report the measurements as “not trusted” after the boot sequence is finished.
The CRTM provides security throughout the boot and run process by extending the root of trust into a chain of trust. The chain of trust provides assurance that the system boot was carried out by trusted firmware and that every application since the boot can be trusted. In a PC, the chain of trust begins by the CRTM measuring and reporting on itself. The CRTM then first measures and reports on the BIOS. The BIOS then measures and reports on the boot loader. The boot loader, in turn, measures the operating system (OS) kernel. The OS then uses the TPM to measure drivers and other applications. In summary, every TCG compliant software module must measure any modules that it calls. With this system, as long as software is measured and the result stored before execution, any unauthorized software cannot hide itself. If unauthorized software is present, it will be reflected in an unexpected hash result stored by the TPM.
The TPM stores hash results in Platform Configuration Registers (PCRs) and a corresponding measurement log in memory external to the TPM. The PCRs contain values representing the sequence of measurements and the measurement log contains a full history of all measurements. The log and PCR values can be used to validate one another. In an example of this process, the CRTM creates a hash of the software about to run and then reports a description of the measured software and the measurement itself to the TPM. The TPM stores the description and measurement in a log. Next the TPM appends the measurement to the value already stored in the appropriate PCR, hashes this new value, and replaces the existing value in the PCR with the new hashed value. A platform's specification may dictate to which PCR a measurement is stored.
In a trusted platform implementing TCG specifications, the hash function is referred to as a SHA-1 function. A PCR contains what may be known as a SHA-1 digest, which is just the value representing the hash of the measured code. PCRs are initialized to zero at boot up. According to TCG specifications, the SHA-1 function concatenates the present value of the SHA-1 digest with the code to be measured. The concatenated value is then hashed and the result is stored in a particluar PCR register. The data processing system then extends a given PCR by concatenating a new data or software with the stored value, hashing the concatenation, and storing the resulting hash value in the PCR. Therefore, as the process of measuring software proceeds, the SHA-1 digest contains the hash of a previously hashed value (i.e., the hash of a hash of a hash, etc.). Through such a sequence, a single PCR value can record a limitless sequence of values. In a TCG system, the value of the SHA-1 digest changes with each hash, but the address size of the SHA-1 digest is always 20 bytes.
A program may ask the TPM to “seal” objects or data to a certain PCR value. The PCR value represents the platform state. Sealing protects storage and allows access only when the platform state as recording in the PCR matches a certain value. Sealed data can only be unsealed if the PCR has a specified value. For example, a developer can seal certain data until after a program loads and runs three software programs, for instance the BIOS, the operating system, and a keyboard driver. After running the BIOS, the operating system, and the keyboard driver, the value in a specific PCR would be the correct value, assuming the system was secure to that point. If the value of the PCR reached the required state, the TPM would release a key to be used by an entity to access the data or code object.
Sealing data to a specific platform state can prevent a breach of confidential data when data storage is not under the designated software or when the data is moved to another platform. If unexpected software was booted or a virus entered the code about to be run, the PCR value will not match and the unseal will fail.
Sealing data to a particular platform state can also be referred to as “locking” the data. For example, a programmer could lock data to a keyboard driver. Only after the keyboard driver was loaded and measured could the data be accessed. This is because only after the keyboard code was measured and loaded would the platform state, as displayed by the PCR registers, be correct for unlocking the data.
Therefore, the TPM of a trusted platform can provide facilities for securely and reliably storing and reporting integrity metrics. However, problems exist because the secure storage is located within the TPM's random access memory (RAM) and is not meant to hold a large quantity of data. As a result, current hardware implementing TCG specifications supports only 16 PCRs. Having only 16 PCRs results in a lack granularity because there are only 16 PCRs to which to seal or lock data. A hypothetical trusted platform with 16 PCRs could call for all drivers to be locked to a value stored in PCR 8 (as in, PCR 8 of sixteen). As the system loaded and ran drivers, the state of PCR 8 would change as its value was hashed with code that was about to run. However, if one driver is changed at all, the value in PCR 8 would not match the sealed value, and the data would not be unlocked. In a circumstance in which new equipment is installed and therefore new drivers are loaded, the ability to seal and unseal data is compromised. What is needed is a more robust method for trusted platforms to seal data to PCRs.